What appears to be the problem as the play opens

Antonio, a merchant, is in a melancholic state of mind and unable to find a reason for his depression. His friends Salerio and Solanio attempt to cheer him up by telling him that he is only worried about his ships returning safely to port. Antonio, however, denies that he is worried about his ships and remains depressed. His two friends leave after Bassanio, Graziano and Lorenzo arrive. Graziano and Lorenzo remark that Antonio does not look well before exiting, leaving Bassanio alone with Antonio.